The present invention relates generally to optical disc data storage systems. More specifically, the present invention relates to optical disc data storage systems which use near field recording techniques.
Optical data storage disc systems are a promising technology for storing large quantities of data. The data is accessed by focusing a laser beam or other light source onto a data surface of the disc and analyzing light reflected from or transmitted through the data surface. In general, in optical storage systems, data is in the form of marks carried on the surface of the disc which are detected using the reflected laser light. There are a number of different optical disc technologies which are known in the industry. For example, compact discs are currently used to store digital data such as computer programs or digitized music. Typically, compact discs are permanently recorded during manufacture. Another type of optical system is a write-once read-many (WORM) system in which a user may permanently write information onto a blank disc. Other types of systems are erasable, such as phase change and magneto-optic (M-O) systems. Phase change systems detect data by sensing a change in reflectivity. M-O systems read data by measuring the rotation of the incident light polarization due to the storage medium.
The above systems require a beam of light to be focused onto a data surface of a disc and recovering the reflected light. Storage density is determined not only by the size of the markings on the data surface, but also by the size of the beam focused on the surface (i.e. resolution). One type of optical element which can be used in conjunction with an objective lens to reduce the ultimate spot size of the light beam is a Solid-Immersion-Lens or SIL. A SIL reduces the beam spot size by virtue of the wavelength reduction which occurs when light is inside an optically dense medium. The SIL is positioned very close to the data surface of the disc and couples to the disc surface via evanescent waves. The use of SILs for data storage is described in U.S. Pat. No. 5,125,750 to Corle et al. which issued Jun. 30, 1992 and in U.S. Pat. No. 5,497,359 to Mamin et al. which issued Mar. 5, 1996. In these optical systems, a laser beam is focused onto the SIL using an objective lens. The SIL is preferably carried on a slider and the slider is positioned close to the disc data surface.
Another type of optical element used for near field recording is an optical probe consisting of a tapered optical fiber. U.S. Pat. No. 5,286,971, issued Feb. 15, 1994 to Betzig et al. describes such a system. The system described in the Betzig et al. patent uses an optical fiber 70 which couples to a probe 20 which, has a tapered profile as shown in FIG. 5 of the Betzig patent. Another technique for near field coupling in optical systems is the use of a pin hole aperture. Such a lens is typically a short waveguide made by selectively doping a glass fiber.
Another type of optical element is a lens formed by an optical fiber having a graded index of refraction. The fiber is used to focus the light rays onto the disc surface and typically operates in the "far-field" regime.
However, there is a continuing need for an inexpensive and easily produced head for optical recording systems which uses near-field coupling.
Further, the use of a solid immersion lens as taught by Corle et al. in U.S. Pat. No. 5,125,750 requires an objective lens that is relatively large and thus requires increased spacing thereby preventing size reductions.